That isnt actually a brexit thing. There always was passport controls as the UK was never part of Schengen.
There will not be sufficient space to handle London-bound passengers at the station, given the more intensive post-Brexit formalities.
What the article should say is 'given the requirement to do full passport and security checks'.
Once EES comes into effect the stamps may not be necessary anyway, but much like fusion, it's perpetually 6 months away.
The main difference is that pre-2021 UK travellers simply had to show travel documents whereas now they have to get them stamped.
On the surface, it isn't clear to me why that makes much of a difference in service time, but the CEO of Eurostar pointed a finger to these changes in January: https://www.bbc.com/news/business-64390979, claiming their trains carry 30% fewer passengers due to station bottlenecks.
And yes, they claim the EES checks should help speed this bottleneck.